The embodiments described herein relate generally to rotary machines and, more particularly, to purge systems for use with turbine fuel supply systems.
In some known dual-fuel turbines, the turbine is powered by burning either a gaseous fuel or a liquid fuel. Such turbines have fuel supply systems for both liquid and gaseous fuels, but, generally most dual-fuel turbines do not burn both gaseous and liquid fuels at the same time. Rather, when the turbine operates with liquid fuel, the gaseous fuel supply is removed from service, and, alternatively, when the turbine operates with gaseous fuel, the liquid fuel supply is removed from service. Moreover, some known turbines burn liquid fuel exclusively and have more than one source of liquid fuel, therefore, such known turbines may require periodic liquid fuel source shifting.
In at least some known industrial turbines, a combustion system may have an array of combustor assemblies, each of which includes at least one liquid fuel nozzle and at least one gaseous fuel nozzle. Generally, in such known turbines, combustion is initiated within the combustor assemblies, downstream from the fuel nozzles. Air from a compressor flows around and through the combustor assemblies to provide oxygen for combustion.
Some known existing turbines that are operable with dual-fuels use gaseous fuel as the primary fuel and use liquid fuel as a backup. During burning of gaseous fuel, the liquid fuel nozzles are normally purged using a purge air system. However, to facilitate readiness for a rapid fuel transfer, static liquid fuel may remain in a portion of the system. During those periods when the liquid fuel system is removed from service, the purge air system operates at a higher pressure at the point of flow communication with the liquid fuel system and, as such, air infiltration into a portion of the liquid fuel system is more likely. Such an operating condition may increase a potential for interaction between fuel and air, and subsequently, may increase carbonaceous particulate formation, sometimes referred to as “coking”. Thin layers of carbonaceous materials are sometimes referred to as “varnish”.
In general, the longer a liquid fuel system remains out of service, the more likely that the static liquid fuel within the turbine compartment will begin to experience carbonaceous particulate precipitation, i.e., coking. Purge air infiltration into the liquid fuel system increases air contact with liquid fuel and the potential for extended air-to-fuel interaction increases as the length of period of time associated with maintaining the fuel system out of service increases. Coking is generally accelerated at temperatures above 93° Celsius (° C.) (200 degrees Fahrenheit (° F.)). Also, liquid fuel carbonaceous particulate precipitation is facilitated at lower temperatures in the presence of oxygen. Considering that some known turbine compartment temperatures operate in excess of 157° C. (315° F.), carbonaceous particulate precipitation is even more likely to occur if infiltrating purge air remains in contact with static liquid fuel for an extended period of time within a heated turbine compartment. Depending on the amount of carbonaceous particulates formation, the potential of having a liquid fuel internal flow passage, including those in the combustion fuel supply valves, nozzles, and fuel nozzle exits, becoming obstructed may increase.
Furthermore, purge air is typically much cooler than the components within the combustor assemblies. Therefore, channeling cool purge air into hot combustor assemblies facilitates unnecessary cooling of the components contained therein, including the liquid fuel nozzles. Such cooling requires additional heat input during subsequent restoration of firing of the combustor assemblies, thereby slowing activities associated restoration of the turbine to service and reducing a thermal efficiency of the turbine.